Frequency modulation detector



May 6, 1947. c. G. SONTHEIMER 2,420,268

FREQUENCY MODULATION DETECTOR Filed Feb. 9, 1945 T111 .Llh d ZZ 70 j47,62.

Eaientecl May 6, i947 FREQUENCY MODULATION DETECTOR Carl G. Sontheimer,Stamford, Conn., assignor to Radio Corporation of America, a corporationof Delaware Application February 9, 1945, Serial No. 577,002

(Cl. Z50- 27) 8 Claims.

My present invention relates generally to detectors of angle modulatedcarrier waves, and more particularly to novel and improved frequencymodulated (FM) carrier wave detectors.

Special problems arise in the detection of FM signals transmitted at theultra-high frequencies over wide band widths. Moreover, it may, forexample, be required in certain ultra-high frequency transmission thatthe FM detector be capable of responding to total frequency changescovering a `band as wide as 12 megacycles (mc.)

or more.

It is an important object of my present invention to provide a detectorof angle modulated carrier waves which is capable of providingmodulation signals of high gain, possesses substantially no interactionbetween its high frequency discriminator network and output connections,and is capable of responding with substantial linearity to relativelywide angle variations of the carrier wave. By angle modulation isgenerically meant FM, or phase modulation (PM), or hybrid modulationspossessing characteristics common to both FM and PM.

Another important object of my invention is to provide an FM detectorusing a double triode tube as a high input impedance detector.

Another object of my invention is to provide in an FM detector of thetype whose discriminator is a pair of sloping filters, an improvementwherein the filters feed a pair of triodes provided with a commoncathode circuit; one of the triodes acting as an innite impedancedetector, while the second triode functions as an anode bend detector.

A more specic object of my invention is to provide a detector offrequency modulated carrier waves, the carrier frequency of which is inthe ultra-high frequency range and is varied over a wide `band by videosignals,

A still more speciiic object of my invention is to provide a reliable,efficient and economical form of detector for FM television signals.

Other features of my invention will best be understood by reference tothe following description, taken in connection with the drawing, inwhich I have indicated -diagrammatically several circuit organizationswhereby my invention may be carried into effect.

In the drawing:

Fig. 1 shows diagrammatically an illustrative embodiment of theinvention;

Fig. 2 illustrates the response characteristic of the present detector;and

Fig. 3 shows a modied form of the invention.

Referring now to the accompanying drawing, wherein like referencecharacters in the several figures designate similar elements, thedetector circuit of Fig. l is explained in connection with a system forreceiving FM television signals.

However, I wish it clearly understood that my invention is entirelygeneric so far as frequency ranges are concerned. Further, there is norestriction on the nature of the modulation, since the invention may,for example, also be used for angle modulated waves (as FM waves) in thepresent 42 to 150 mc. band assigned to FM transmission. In theillustrative case given herein, the carrier may be chosen from a rangeup to 100 mc. or more. The video or picture signals may vary or deviatethe carrier frequency to a maximum of 6 mc., on either side of thecarrier or center frequency of the radiated wave. The total frequencychange may exceed, or be less than, 12 mc., if desired.

Those skilled in the art of radio communication are fully aware of theVarious methods for collecting the radiated FM waves, selectivelyamplifying the collected waves at carrier frequency, reducing the meanfrequency of the selectively amplified FM waves to a suitable lowerfrequency value. Let it be assumed, for example, that the FM wave energyhas been reduced to a center or mean frequency of mc..prior todetection, Va1- though no limitation is imposed on my invention b-yvirtue of this illustrative assumption. It may be further .assumed thatit is desired to derive from the 70 mc. wave energy video signals whichcorrespond with frequency variations of the '70 mc. wave over a band of12 mc.

As is Well known to those skilled in the art of FM communication, at thetransmitter the modulation signals cause the carrier frequency to varyor Vchange in accordance with the amplitude of the modulation, whilemodulation frequencies per se determine the rate of the frequencychange. Fig. 2 shows, in ideal form, the nature of the response desiredat the FM detector. Curve a depicts the FM signal at the high frequencydiscriminator input circuit of the FM detector The wave varies atmaximum deviation from amean or center frequency Fc of '70 mc.V tofrequencies of 64 mc. and 76 mc. on either side thereof. Curve b showsthe response characteristic of the FM detector. Frequency changes areplotted against rectified voltage to secure curve b. As the curve avaries between 64 and '76 mc., the rectified voltage output of thedetector varies from maximum positive voltage to maximum negativevoltage. At 70 mc. the rectified voltage output is zero, and between thepeaks of curve b it is desirable to have the curve as linear aspossible. The rectified voltage variations correspond to the modulatingsignal variations at the transmitter. It will be understood that thepeaks of curve b could actually beat vsubstantially'less than 64 mc.,and at greater than 76 mc. to provide a pass band at the detector inputcircuit in excess of 12 mc. v

The characteristic shown ideally as curve b in Fig. 2 is provided, inaccordance with my invention, by using a circuit as shown in Fig. 1preferably employing a tube I of the twin triode type. The tube may beof any well known and suitable design such as 6J6. Separate tubes may beused if desirable. The triode comprising cathode 2, control grid 3 andanode or plate 4 may for convenience be called the input triode, and thetriode comprising cathode 5, control grid 6 and anode or plate 'I may betermed the output triode. Cathodes 2 and 5 are connected in common toground through the resistor S. The resistor 8, which may have amagnitude of between 500 to 1000 ohms and up to 3000 ohms if desired, isbypassed for carrier frequency currents, but not for currents ofmodulation frequencies. Numeral 9 indicates the bypass condenser,represented by dash lines, as distributed capacity of the leads. Aphysical condenser could be used at 9, but at high frequencies of theorder of 70 mc. and above the distributed capacity will usually sulce.

Plate 4 is connected to a suitable positive potential point (+B) of adirect current source. A filter resistor I is inserted in the platesupply lead. The plate 4 is bypassed to ground by condenser II for theradio, i. e., carrier frequency, currents as well as video, modulationfrequency components. Plate 'I is connected to the +B terminal throughthe output resistor I2, plate I being bypassed to ground by condenser I3for radio frequency currents only. Resistor 8 and plate I cannot bebypassed for video frequencies without losing the desired video signal.An infinite impedance detector works best, in general, with its platevbypassed to radio frequency and video currents. Hence, the plate 4 iscompletely bypassed. The load resistor I2 could have a magnitude of from1000 to 2000 ohms for a video band of 6 mc.

The control grids 3 and 6 are connected to suitable points of anydesired type of discriminator input circuit. My invention is not limitedto the specific discriminator circuit shown in Fig. 1, which is of thetype disclosed in U. S. Patent No. 2,057,640 to Conrad. This inputcircuit comprises a tuned primary circuit I4 resonated to Fc (70 mc.).The circuit I4 may be located in the plate circuit of a prior 70 mcamplifier tube, or it may be the output circuit of an amplitude limitertube. The shunt resistor I5 provides damping for suitably broadening theresponse curve of the discriminator network.

Secondary circuit I6, magnetically coupled as at Mi to primary circuitI4, is tuned to a frequency greater than Fc. For example, circuit I6could be tuned to a frequency somewhat greater than 76 mc. Dampingresistor I'I shunts circuit I6 to broaden the response thereof.Secondary circuit I8 is magnetically coupled to primary circuit I4, asat M2, and is tuned to a frequency less than Fc. The frequency ofcircuit I8 could be as much below 64 mc., as the frequency of circuit ISis above '76 mc. Damping resistor I9 shunts tuned circuit I8.

The high alternating potential side of circuit I6 is connected by lead20 to grid B, while the high potential side of circuit I8 is connectedto grid 3. The low potential sides of both secondary circuits I6 and I8are grounded. Operating bias is provided for each of grids 3 and 6 bythe voltage drop across resistor 8. The operating bias in this circuitmay approach the cut-off bias for the tube, and is in any event greaterthan the the circuit I4 by equal frequency values.

bias that would be used with the same tube as I an amplifier.

The radio frequency voltages at points c and d will vary in polarity andmagnitude relative to ground. The variation will depend on the sense andamount respectively of frequency deviation of the applied FM signalenergy relative to Fc. The action of the discriminator circuit is wellknown. At the frequency Fc of the FM wave energy the voltage at point cis equal to the voltage at point d, because the circuits I6 and I8 aremistuned from In response to a deviation of the signal energy to itsmaximum frequency approaching the frequency of circuit I6 the signalvoltage at point c will be a maximum, while the voltage at point d willbe a minimum. The reverse situation takes place in response to afrequency deviation to minimum.

frequency approaching the frequency of circuit I8.

To explain the functioning of the triode sections of tube I, let it beassumed that the frequency of the signal energy at primary circuit I4 at76 mc. In that case the signal voltage applied to grid 6 is a maximum.This will cause a large flow of rectified current through the resistor 8due to maximum plate current flow in the output triode 5, 6, 'I of tubeI. It will, also, be noted that the signal voltage at point d is aminimum, and the rectified plate current flow due to the input triode 2,3, 4 will be small. Thus the output triode acts as an anode benddetector, or plate rectification detector, and the anode end of resistorI2 will have minimum positive polarity (or be relatively most negative)as indicated by the lower peak of curve b in Fig. 2.

If, now, the signal frequency at circuit I4 deviates to 64 inc., thesignal voltage at point d is a maximum. This will increase the platecurrent flow of the input triode to a maximum, and cause the cathode endof resistor 8 to become highly positive relative to ground. With thisapplied frequency, only a relatively small radio frequency voltageappears between the grid 6 and ground; rectified current in the outputtriode due to voltage on grid 6 is, therefore, small. As a consequence,the anode end of resistor I2 rises to a maximum positive potential, asindicated by the upper peak of curve b in Fig. 2.

The negative bias on grids 3 and 6 is such that neither of these gridsis ever driven positive by the signal energy. Hence, the input triodeacts in the manner of an infinite impedance detector driving the outputtriode through the cathode, as in a grounded grid amplifier. Sincecathode 5 is bypassed to ground for all radio frequency currentcomponents, no radio frequency voltage appears across this element. Thecombination of the input triode section acting as an infinite impedancedetector, and the output triode as a grounded grid amplifier, causes anincrease in radio frequency voltage at point d to produce a rise inpotential at the output plate. There is only second order interactionbetween the discriminator input network and the video output circuit.The latter comprises the video output connection 2i and couplingcondenser 22. Relatively high gain is provided due to the amplificationprovided by the output triode of tube I.

It is pointed out that with my present invention less interactionbetween the high frequency discriminator circuit and the video outputcircuit is secured than would be the case with known types of FMdetector circuits. Both triode detectors in this circuit present aninfinite grid impedance to the applied signal in that the radiofrequency voltage draws no current to the electrode where it is applied.This should be contrasted to the action of an FM detector employingdiodes. I-Ience, the tube impedance is much higher than the impedance ofthe damping resisters il and i9 used to broaden the discriminator inputcircuits. The value of these damping resistors il and I9 is thus notappreciably affected by the parameters chosen for the video circuits, e.the output resistor i2. This would not be the case of an FM detectoremploying diodes in a wide band system. Improved linearity may beachieved since resistor i2 and the damping resistors il and i9 may bechosen independently for the best performance of their individualfunctions.

At frequency Fc the grids 3 and will provide plate currents throughresistor 8 such that the voltage of the anode end of resistor l2 isintermediate between the peak values of curve b of Fig. 2. Hence, theoutput voltage at resistor i2 is proportional to the difference inexcitation between the pair of resonant circuits EE and i8. The systemhas a response characteristic balanced at Fc, and providing asingle-ended output. The portion of the characteristic b between thepeaks may be kept highly linear by proper radio frequency circuitdesign.

In Fig. 3 I have shown a modication of the FM detector circuit, whereinthe discriminator input network is of the type disclosed and claimed byS. W". Seeley in his U. S. Patent No. 2,121,103, granted Llune 2l, i938.While the construction and functioning of this type of discriminatorcircuit are substantially different from those of the Conraddiscriminator circuit shown in Fig. l, yet the resultant signal voltagesproduced at points c and d are respectively the same as at therespective points in Fig. l. In the case of Fig. 3 it is assumed thatthe primary resonant circuit id is located in the plate circuit of thelast intermediate frequency (I. F.) amplifier of a superheterodynereceiver suitably constructed to receive the ultra-high frequencysignals. Circuit ifi is, of course, tuned to the operating I. F. value,say, for example, 70 rnc. The low alternating potential side of circuitid is established at ground potential by means of condenser ifi forsignal frequencies.

rEhe secondary circuit 3l is tuned to the operating I. value and themidpoint of its coil 3i is connected by direct current blockingcondenser 3@ to the high potential side of circuit id. Magnetic couplingM3 provides sufficient coupling between the circuits i4 and 3i toprovide a band pass characteristic such as efliciently to pass a band offrequencies in excess of l2 mc. IThe shunt damping resistors i5 and 32aid in broadening the pass band of these circuits.

The opposite ends of coil 3l are connected to ground by means of gridleak resistors 33 and 34, these ends being designated as points d and crespectively. As in the case of Fig. 1, control grids t and 6 of thetwin triodes are connected respectively to points d and c. The circuitsassociated with tube i are generally the same as described in connectionwith Fig. l, with the exception of the common cathode circuit of thetube. It will be noted that the cathodes f2 and 5 are connected togetherby means of a radio frequency choke coil 35. The purpose of this chokeis to prevent coupling at radio frequency between the two sections ofthe tube, and here serves to augment the effect of bypassing thecathodes to radio frequency. The resistor'is connected from cathode 5 toground, and the resistor is shunted by the condenser 9 which bypassesthe radio frequency currents only. Condenser 3B, also, bypasses radiofrequency currents only, and is connected from cathode 2 to ground.

The circuit of Fig. 3 functions in the same manner as the circuit ofFig. l. In general, it can be stated (more specific reference being madeto the Seeley patent) that the resultant signal voltages at points c andd are produced by virtue of signal voltages induced in the secondarycircuit 3i through the magnetic coupling M3 and the direct connectionwhich includes condenser 30. That is to say, the signal voltage atprimary circuit I4 is directly applied at the midpoint of coil Si', andthis signal voltage is applied in parallel relationship to grids 3 and6. On the other hand, the signal voltage induced in circuit 3l throughthe magnetic coupling is applied in push-pull relation to grids 3 and 6.Hence, the resultant signal voltage applied t0 each of grids 3 and 6 isthe vector sum of a pair of voltages in normal phase quadrature relationat the operating I. F. value. For frequencies other than mean or centerfrequency (the I. F. value) the vector voltages at the grids will beunequal, because of the fact that the phase shift introduced for suchoff-frequencies by the magnetic coupling M3 differ from the normalshift. It will, therefore, be seen that at center frequency the signalvoltages at points c and d are equal, Whereas for frequencies differentfrom center frequency the polarity and magnitude relation of thevoltages will depend upon the direction and degree respectively offrequency deviation of the signal energy at circuit Ul with respect toFc. This, of course, is precisely the voltage relations existing atpoints c and d in Fig. 1.

The discriminator input circuit may be so arranged as to provide for anincrease in signal voltage at point c when the signal energy deviates tofrequencies in excess of Fc, While the signal voltage at point d becomesrelatively greater when the frequency deviation is less than Fc. Here,again, the relations would be the same as in the case of Fig. 1.

Summarizing the features of my present invention, there is secured ahigh radio frequency impedance thereby permitting a choice of dampingresistors at the discriminator input circuit without reference to videorequirements; video voltage obtained from plate 'I to ground isdeveloped across a single load resistor I2 With a relatively lowshunting capacity (as small as 1.3 mmf. (micro-micro-farads) for a twintriode tube of the 6J6 type); much higher gain is secured than in thecase Where diodes are employed; greater linearity is provided becausethe output resistor l2 may be specifically chosen` for that purpose andis not influenced by required` damping at the input circuit; moreover,higher linearity can be obtained in the present circuitv with a lowervalue of resistor l2 than would be the case if diodes were employed, itbeing pointed out that a low-magnitude resistor is importantV in a wideband video circuit.

While I have indicated and described a system for carrying my inventioninto effect, it Will be apparent to one skilled in the art that myinvention is by no means limited to the particular organizations shownand described, but that many modifications may be made without de.parting from the scope of my invention.

What I claim is:

1. In a detector of angle modulated carrier Waves, a pair of electrondischarge devices each including at least a cathode, control electrodeand anode, a resistive impedance connecting the cathodes of said devicesto ground, means applying a positive direct current voltage to the`anodes of both devices, an output resistor of relatively ioiv magnitudein circuit with only one of the anodes,.a modulation signal voltageoutput connection from the anode end of saidoutput resistor, andmeansfor varying the control electrodes of saiddevices with respectivesignal voltages derived from said Waves and'varying in relative polarityand magnitude in accordance With the angle modulation of said Waves.

2; In a detector of angle modulated carrier waves,4 a pair of electrondischarge deviceseach including at least a cathode, controll electrodeandanode, a resistive impedance connecting the cathodes of saiddevicesto ground, means applying apositive direct current voltage to the anodesofv both devices, an output resistor in circuit With only one of theanodes, means bypassing from the second. anode currents of carrier andmodulation signal frequencies, a modulation voltage output connectionfrom the anode end of said output' resistor, and means for varying thecontrol electrodes of said devices Withrespective signalvoltages derivedfrom saidwaves and varying in relative polarity and magnitude inaccordance with the angle modulation of said Waves.

3. In a detector of angle modulated carrier waves, a pair of electrondischarge devices each including at least. a cathode, control electrodeand anode, a resistive impedance connecting the cathodes of said devicesto ground, means applying a positive direct current voltage to theanodes of both devices, an output resistor in circuit with only one ofthe anodes, a modulation signal voltages output connection from theanode end of said output resistor, means for varying the controlelectrodes of said devices with respective signal voltages derived fromsaid Waves in accordance with the angle modulation of said Waves, andmeansbypassing each of said'resistive impedance and one anode forcurrents of carrier frequency.

4. InY combination with a source of a pair of high frequency voltageswhose relative phases andimagnitudes vary from a predetermined relation,means for providing a rectified voltage responsive to said variation,said means comprising a pair of electron discharge devices each havingat least a cathode, control grid'and plate, a common resistive impedancefor both cathodes adapted to be traversed by the space currents of bothdevices, circuit elements operatively associated with the electrodes ofone devce to render it operative as an infinite impedance detector,meansV applying one of said voltages to the control grid .of said onedevice, circuitelements operativelyk associated with the second devicetorender it operative as a plate rectification device, means applyingthe second voltage to the control grid of the second device, and meansfor, utilizing said rectied voltage. developed in the plate circuit ofthe second device.

5. In combination with a source of a pair of high frequency voltagesWhose relative phases and magnitudes vary from a predetermined relation,means for providing a recti'ed voltage responsive to said variation,said means comprising a pair of electron discharge devices each havingYat least a; cathode, control grid and plate, a

common resistive impedance for both cathodes adapted to be traversed bythe space currents of both devices, circuit elements operativelyassociated with the electrodes of one device to render it operative asan infinite impedance detector, means applying one of said voltages tothe control gridof said one device, circuit elements operativelyassociated with the second device to render it operative as a platerectcation device, means applying the second. voltage to the controlgrid of the second device, and means for utilizing said rectifiedvoltage developed in the plate circuit of the second device, said sourceincluding damping resistive impedance of high value, and said platecircuit of said second device including resistive impedance ofrelatively low magnitude.

6. In a detector of frequency modulated carrier Waves, a pair ofelectron discharge devices each including at least a cathode, controlelectrode and anode, a resistor connecting the cathodes of said devicesto ground, a source of positive direct current voltage, a rst resistorin circuit between said source and the anode of one device, an outputresistor in circuit between said source and the second one of lcheanodes, a modulation signal voltage output connection from the anode endof said output resistor, means for varying the control electrodes ofsaid devices with respective signal voltages derived from said waves,means bypassing the said anode of said one device for carrier andmodulation frequencies, and means bypassing said cathode resistor andsecond anode for carrier frequencies only.

7. In a detector of frequency modulated signals, a pair of electrondischarge devices each including at least a cathode, control electrodeand anode, a resistive impedance connecting the cathodes of said devicesto ground, means applying a positive direct current voltage to theanodes of botlidevices, an output resistor in circuit with only one ofthe anodes, means bypassing the anode for currents of carrier andmodulation frequencies, a modulation voltage output connection from theanode end of said output resistor, a signal input circuit constructed asa discriminator for varying the control electrodes of said devices withrespective signal voltages derived from said signals, means highlydamping the discriminator circuit to give it a relatively wide passband, and said output resistor having a relameans bypassing the secondanode for high frequency and modulation currents, a modulation voltageoutput connection from the anode end of said output resistor, adiscriminator input circuit of relatively high damping, saiddiscriminator circuit providing for the control electrodes of saiddevices respective signal voltages derived from said waves related inpolarity and magnitude in accordance with the angle modulation of saidWaves, means bypassing said one anode for current of highl frequencyonly, and said output resistor being of a relatively low resistivemagnitude.

CARL G. SONTHEIMER;

